Osmotic energy

ABSTRACT

A process for driving a prime mover, said process comprising a) positioning a selective membrane between a liquid and a solution having a higher osmotic potential than the liquid, such that the solution becomes pressurised by the influx of liquid across the membrane, b) using the pressure generated in the solution to drive a prime mover, c) recovering the solution, d) separating at least some of the solvent from the solution to form a residual product, and e) recycling the separated solvent and/or the residual product of step d) to step a).

The present invention relates to a process for driving a prime mover.The present invention also relates to an apparatus for driving a primemover, for example, for generating hydraulic power.

When a dilute aqueous solution (low osmotic potential, low entropy) isseparated from a concentrated aqueous solution (high osmotic potential,high entropy) by a semi-permeable membrane, water from the diluteaqueous solution will flow across the semi-permeable membrane to dilutethe concentrated aqueous solution. The phenomenon is known as osmosis.

U.S. Pat. No. 3,978,344 describes a process in which osmotic energy isconverted into other forms of energy. Specifically, this referencedescribes a process in which a semi-permeable membrane is used toseparate a body of freshwater from a body of seawater. The difference inosmotic potential between the freshwater and the seawater causes waterto pass through the membrane into the seawater, resulting in an increasein hydrostatic pressure on the seawater side of the membrane. Theseawater may be ejected as a pressurised stream, which may be used torotate a turbine coupled to an electrical generator.

The passage of freshwater through the semi-permeable membrane results indilution of the seawater, which eventually limits the production ofenergy from a given body of seawater. Thus, the diluted seawater must bereplenished periodically or continuously with a fresh seawater.

According to the present invention, there is provided a process fordriving a prime mover, said process comprising

-   -   a) positioning a selective membrane between a liquid and a        solution having a higher osmotic potential than the liquid, such        that the solution becomes pressurised by the influx of liquid        across the membrane,    -   b) using the pressure generated in the solution to drive a prime        mover,    -   c) recovering the solution,    -   d) separating at least some of the solvent from the solution to        form a residual product, and    -   e) recycling the separated solvent and/or the residual product        of step d) to step a).

In one embodiment, the residual product of step d) is recycled to thesolution of step a). Alternatively or additionally, the separatedsolvent may be recycled to the liquid side of the membrane in step a).

The influx of liquid across the selective membrane generates pressure(e.g. hydrostatic pressure) the solution. The pressurised solution fromstep a) may be used directly to drive the prime mover. Alternatively,the solution from step a) may be recovered and some of the solvent ofthe recovered solution may be removed. The resulting concentratedsolution may be used to drive the prime mover before being recycled tostep a). Alternatively or additionally, the solvent separated from thesolution may be used to drive the prime mover. In all instances, atleast some of the pressure generated in the solution in step a) is usedto drive the prime mover. optionally, the generated pressure in thesolution may be used to pump the solution for subsequent processing.

Any suitable selective membrane may be used in step a). The membrane mayhave an average pore size of 1 to 60 Angstroms, preferably, 2 to 50Angstroms, more preferably, 5 to 40 Angstroms, for example, 10 to 30Angstroms. In one embodiment, the membrane has an average pore size of12 to 25 Angstroms.

The average pore size of the membrane is preferably smaller than thesize of the solutes in the solution.

Advantageously, this prevents or reduces the flow of solute acrossmembrane by diffusion, allowing liquid to flow across the membrane alongthe osmotic (entropy) gradient. The flux of liquid across the membraneis influenced by the pore size of the membrane. Generally, the largerthe pore size, the greater the flux.

Suitable selective membranes include integral membranes and compositemembranes. Specific examples of suitable membranes include membranesformed of cellulose acetate (CA) and membranes formed of polyamide (PA).Preferably, the membrane is an ion-selective membrane. Conventionalsemi-permeable membranes may also be employed.

The membrane may be planar or take the form of a tube or hollow fibre.If desired, the membrane may be supported on a supporting structure,such as a mesh support. The membrane may be corrugated or of a tortuousconfiguration. The prime mover may be any suitable device, which issuitable for converting energy in the solution into mechanical power.Suitable prime movers include rotary prime movers, such as turbines.Thus, the prime mover may be used to generate power.

Alternatively, the prime mover may be or form part of a pressureexchange system. Thus, the prime mover may also be used to transferenergy from the pressurised solution to another fluid. Examples ofsuitable pressure exchange systems are described in U.S. Pat. No.4,887,942, U.S. Pat. No. 5,338,158, U.S. Pat. No. 5,988,993 and U.S.Pat. No. 6,540,487. The pressure exchange system may comprise a housinghaving a body portion with end elements at opposite ends of the bodyportion. A rotor may be positioned in the body portion in substantiallysealing contact with the end plates. The rotor may be provided with atleast one channel extending longitudinal from one end of the rotor tothe opposite end of the rotor with an opening at each end. In use, thechannel(s) provide alternate hydraulic communication between a highpressure liquid, such as the pressurised solution from step a), and alow pressure liquid to transfer pressure between the liquids.

In the process of the present invention, a selective membrane ispositioned between a liquid and a solution. The solution has a higherosmotic potential than the liquid. Thus, the total dissolved salt (TDS)concentration of the solution is typically higher than that of theliquid.

The difference in osmotic potential between the liquid and the solutioncauses solvent to pass across the selective membrane from the side oflow osmotic potential (i.e. low solute concentration or low entropy orhigh solvent concentration) to the side of high osmotic potential (i.e.high solute concentration or high entropy or low solvent concentration).This influx of liquid leads to an increase in pressure in the solution.For example, the pressure of the solution may be increased from 10⁵ to10⁷ Pa to a pressure of 1.1×10⁵ to 5.0×10⁷ Pa due to the influx ofliquid. In a preferred embodiment, the pressure of the solution may beincreased from 10⁵ to 10⁷ Pa to a pressure of 1.5×10⁵ to 2.5×10⁷ Pa dueto the influx of liquid.

Step a) of the process of the present invention may be carried out in ahousing. The housing is preferably provided with an inlet forintroducing the liquid to one side of the selective membrane and anoutlet for removing pressurised solution from the opposite side of themembrane. In a preferred embodiment, the influx of liquid into thesolution is sufficient to eject the solution from the housing at apressure of 1.1×10⁵ to 5.0×10⁷ Pa, preferably, 1.5×10⁵ to 2.5×10⁷ Pa.The outlet of the housing may be sized to ensure that the solution isejected at appropriate pressures. Additionally or alternatively, anozzle (a pressure regulator) may be coupled to the outlet to adjust thepressure of the solution accordingly.

The pressurised solution may be removed from the housing, for example,as a pressurised stream. The pressure generated in the solution may beused to drive a prime mover. The solution may be introduced directly tothe prime mover to drive the prime mover, for example, to generatepower. Alternatively, the pressure generated in the solution may betransferred to another liquid via a pressure exchange system. Thepressurised solution from step a) may be used directly in step b) todrive the prime mover. Alternatively, it may be possible to carry outsteps c) and d) of the process prior to step b).

The pressure generated in the solution may be sufficient to eject thesolution from the housing to an elevated height of, for example, 10 to2500 m, preferably, 50 to 1500 m. Thus, the solution may be introducedto the prime mover from an elevated height. In this way, at least partof the potential energy of the solution is converted into mechanicalenergy. The mechanical energy of the prime mover may be subsequentlyconverted into other forms of energy, such as electricity and/or heat.In certain embodiments, it may be desirable to store the solution at anelevated location, prior to introducing the solution to the prime mover.

In step c), the solution is recovered. Solvent is then removed from therecovered solution (step d). This solvent removal step may be carriedout by any suitable solvent removal/separation method. Thermal and/ormembrane separation steps may be employed. A combination of one or morethermal separation steps and/or one or more membrane separation stepsmay be employed.

Examples of suitable thermal separation techniques include evaporation,distillation and crystallization. Evaporation may be carried outnaturally, for example, by allowing the solvent to evaporate in airunder ambient conditions. Alternatively, evaporation may be carried outin a cooling tower. Suitable distillation methods include multi-stageflash distillation (MSF), multi-effect distillation (MED), mechanicalvapour compression (MVC) and rapid spray desalination.

In multi-stage flash distillation, the solution is introduced into aseries of tubes and heated to an elevated temperature. The heatedsolution is then introduced into an evaporation chamber and subjected toa pressure below its vapour pressure. The sudden reduction in pressurecauses boiling or flashing to occur. The flashed vapours are separatedfrom the salty residue by condensation on the tubes of the incomingsolution streams. A series of evaporation chambers is typicallyemployed. Thus, the evaporation or flashing step occurs in multiplestages.

Multiple effect distillation takes place in a series of effects and usesthe principle of reducing the ambient pressure in the various effects.This permits the solution to boil in a series of stages without the needfor additional heat to be supplied after the first effect.

In multiple effect distillation, the solution may be preheated andsprayed onto the surface of evaporator tubes as a thin film of liquid.The tubes are heated by passing a steam through the tubes. On cominginto contact with the heated surface of the tubes, the sprayed liquidevaporates. This vapour is used to heat the evaporator tubes of the nexteffect and the transfer of heat causes the vapour in the tubes tocondense. By evaporating and condensing the solution in this manner, thesolvent from the solution may be recovered.

The efficiency of the multiple effect distillation step may be increasedby compressing the vapour of at least one of the effects. Thecombination of multiple effect distillation and compression is known asMED-thermo compression.

Mechanical vapour compression (MVD) may also be used to remove solventfrom the solution. In mechanical vapour compression, vapour from avessel is typically extracted and then condensed by compression in atube located within the vessel. The compression and condensation stepgenerates heat, which heats the walls of the tube. When solution issprayed onto the surface of the tube, it evaporates generating morevapour. By repeating the extraction, compression and condensation steps,further solvent may be recovered from the solution.

Rapid spray desalination (RSD) may also be used to remove solvent fromthe solution. In a typical rapid spray desalination process, air isblown across a heating element into an evaporation chamber. As theheated air moves along the evaporation chamber, a nebulized solution of,for example, brine, is injected into the evaporation chamber. The movingvapour and brine droplets pass through a mechanical filter, which trapsthe brine droplets, allowing the pure vapour phase to pass on towards acondenser. The brine droplets may be periodically flushed from thefilter.

As mentioned above, crystallization methods may also be employed toseparate solvent from the solution. Crystallization may be affected tocrystallize the solvent or solute out of solution.

Crystallization may be carried out by cooling the solution to, forexample, the freezing point of the solvent. This causes at least some ofthe solvent in the solution to crystallize. This crystallized solventmay then be removed. Crystallization may be preferred in cool climates,where the low ambient temperatures may be used to reduce the temperatureof the solution to effect crystallization.

Alternatively, a thermal separation column may be employed to affectcrystallization. For example, the solution may be cooled in a thermalseparation column such that at least some of the dissolved solutesprecipitate out of solution. These precipitates may collect at thebottom of the column and recovered, leaving the solution at the top ofthe column with a reduced solute concentration. Advantageously, thesolution may be formed using a salt having a solubility that issensitive to temperature variations. Preferably, such salts readilyprecipitate out of solution at low temperatures. Examples of such saltsinclude hydrogenphosphates such as disodium hydrogenphosphate(Na₂HPO₄.12H₂O).

In one embodiment, the solution from step a) may be transferred to anelevated height (e.g. top of a mountain) where the ambient temperatureis i) low enough to crystallize the solutes species in the solution orii) below the freezing point of the solution to crystallize the solvent.This causes separation of the solution into two portions. One portionhas a low solute concentration, whilst the other portion has a highersolute concentration. Each of these solutions may be returned to groundlevel so that the potential energy of the solutions may be used to drivethe prime mover. These solutions may be recycled to step a).

Suitable membrane methods for separating solvent from the solutioninclude ion-exchange, electro-dialysis, electro-dialysis reversal,nanofiltration and reverse osmosis. When membranes are used, they shouldbe able to withstand the high pressures generated in the system.

The thermal energy required to drive the solvent removal step may beprovided by a number of sources. For example, the thermal energy may beprovided by the surroundings (e.g. evaporation at ambient temperature),geothermal sources and/or solar energy. Evaporation at ambienttemperature may be favoured in hot climates. It may also be possible toinduce solvent removal by passing air, for example, dry warm air, overthe solution, so as to effect evaporation of the solvent. Alternativelyor additionally, the excess heat from an industrial process (e.g. apower station, a refinery, chemical plant) may be used to drive thesolvent removal step. In other words, the solvent removal step of thepresent invention may be used to remove excess heat from an industrialprocess.

In a further embodiment, the thermal energy required to drive thesolvent removal step may be provided by the combustion of a fuel, suchas oil, wood, peat, bushes, grass, straw, natural gas and coal. Wasteproducts may also be incinerated to provide the thermal energy requiredfor the solvent removal step.

In yet a further embodiment, the thermal energy required to drive thesolvent removal step may be provided by biological processes, such asthermogensis and fermentation.

In another embodiment, the thermal energy required to drive the solventremoval step may be provided by the compression and decompression of gas(e.g. air). When a gas expands isotropically at a given temperature, itsfinal temperature at the new pressure is much lower. The resulting coldgas, can be used as a refrigerant, either directly in an open system, orindirectly by means of a heat exchanger in a closed system. Conversely,the compression of gas causes the temperature of the gas to increase.The heat of compression can be used to heat the solution and/orevaporate the solvent.

In yet another embodiment, the thermal energy required to drive thesolvent removal step may be provided by wind power. Wind power can beused to compress air and the heat of compression can be used to heat thesolution and/or evaporate the solvent. The air may then be decompressedand the cooling effect of the decompression can be used to cool thesolution and/or condense the vapour. The use of air as a coolant isbased on the principle that, when a gas expands isotropically at a giventemperature, its final temperature at the new pressure is much lower.The resulting cold gas, in this case air, can then be used as arefrigerant, either directly in an open system, or indirectly by meansof a heat exchanger in a closed system.

The sources of thermal energy discussed above may be particularly usefulfor removing solvent by evaporation/distillation.

Once solvent is removed from the solution, a residual product isproduced. This residual product is preferably recycled to a solutionthat is suitable for use in step a). For example, the residual productproduced in step d) of the process may be recycled to the solution ofstep a) of that process. Alternatively, when more than one of theprocesses of the present invention are carried out (e.g. concurrently),it is possible to recycle the residual product of one of the processesto step a) of another of the processes.

The process of the present invention may be carried out continuously,reducing or eliminating the need for replacing or replenishing thesolution of step a) with fresh solution. It is also not necessary to addfresh solute to the solution, although this may be desirable in someinstances.

The residual product may take the form of a solid product or aconcentrated solution. Where the residual product is a solid product,the solid product may be added to the solution of step a) to increasethe solute concentration of the solution of step a). This can help tomaintain the difference in solute concentration between the two sides ofthe membrane, and ensure that the flow of liquid across the membraneoccurs at a sufficient rate.

Where the residual product is a solid product, the solid product mayalso be diluted with solvent to produce a concentrated solution. Thisconcentrated solution can be introduced to the solution of step a). Byadjusting the concentration of this solution accordingly, the soluteconcentration of the solution of step a) may be maintained at desiredlevels. This can help to maintain the difference in solute concentrationbetween the two sides of the membrane, and ensure that the flow ofliquid across the membrane occurs at a sufficient rate.

When the residual product is a concentrated solution, the concentratedsolution may be introduced to the solution side of the selectivemembrane of step a). In certain embodiments, it may be necessary toalter the concentration of the concentrated solution prior to use, forexample, by adding more solvent or solute to the solution. By adjustingthe concentration of the solution accordingly, the solute concentrationof the solution may be maintained at desired levels. This can help tomaintain the difference in solute concentration between the two sides ofthe membrane, and ensure that the flow of water across the membraneoccurs at a sufficient rate.

The solvent removed in the solvent removal step may be recovered andrecycled, for example, to a liquid that is suitable for use in step a).The solvent removed in step d) of the process may be recycled to step a)of the process, or, alternatively, when a plurality of processes of thepresent invention are carried out (e.g. concurrently), the solventremoved in step d) of one particular process may be recycled to step a)of another process.

Alternatively or additionally, the removed solvent may be discarded orused for other purposes. In one embodiment, the liquid is seawater andthe solution is an aqueous solution. Thus, the solvent removed from thesolution in step d) is water. This water may be used for a number ofapplications, including agricultural, industrial and domesticapplications (e.g. as drinking water). Thus, in this embodiment of thepresent invention, the process of the present invention may be used todesalinate seawater.

It may be possible to replenish or to replace the liquid of step a) withfresh liquid, for example, periodically or continuously.

The liquid employed in step a) of the process of the present inventionis preferably water or an aqueous solution. For example, the liquid maybe seawater, freshwater (e.g. from rivers, lakes and undergroundsources) and brackish water. Grey water streams, for example, wastewashing water (e.g. laundry) and streams from gullies, may also be used.Thus, the liquid may contain impurities that are typically found inwater from these sources. For example, the liquid may contain dissolvedsalts, such as metal or ammonium salts. Examples of salts that may bepresent include fluorides, chlorides, bromides, iodides, sulphates,sulphites, sulphides, carbonates, hydrogencarbonates, nitrates,nitrites, nitrides, phosphates, aluminates, borates, bromates, carbides,chlorides, perchlorates, hypochlorates, chromates, fluorosilicates,fluorosilicates, fluorosulphates, silicates, cyanides and cyanates.Preferably, salts of alkali and/or alkali earth metals are employed.Examples of such metals include, but are not limited to, lithium,sodium, potassium, magnesium, calcium and strontium. In one embodiment,the liquid is seawater and, therefore, includes sodium chloride in aconcentration of at least 3 weight %.

In an alternative embodiment, the liquid may be an effluent from anindustrial or agricultural process.

The solute concentration (i.e. TDS) of the liquid may be 0 to 40 weight%, preferably, 0.0 to 6 weight %.

During osmosis, at least some of dissolved solutes and suspendedimpurities in the liquid will be prevented from flowing across themembrane. Preferably, all dissolved solutes/impurities will remain onthe liquid-side of the membrane, allowing the liquid to flow across themembrane to dilute the solution on the other side of the membrane. Thus,where the liquid is a solution of a solute dissolved in a solvent, thesolute is preferably prevented from flowing across the membrane,allowing the solvent to flow across the membrane to dilute the solutionon the other side. Specifically, where the liquid is an aqueoussolution, dissolved solutes and/or suspended impurities in the solutionare preferably prevented from flowing across the membrane, allowingwater to flow across the membrane to dilute the solution on the otherside.

The solution may be formed of an organic and/or inorganic solvent.Suitable organic solvents include hydrocarbons, such as aliphatic andaromatic hydrocarbons. Mixtures of organic solvents may be employed. Thehydrocarbons may be straight chain, branched and/or cyclic. Examplesinclude, but are not limited to, alkanes, alkenes and alkynes. Thehydrocarbons may be substituted with one or more heteroatoms, forexample, fluorine, chlorine, bromine, iodine, oxygen, sulphur, nitrogen,and/or phosphorus atoms. In one embodiment, oxygenated hydrocarbons,such as aldehydes, ketones, carboxylic acids, ethers, esters, alcoholsand/or their derivatives may be employed. For example, glycol ethers andglycol ether esters may also be employed. Alternatively or additionally,halogenated solvents, such as chlorinated, brominated and/or fluorinatedhydrocarbons may be employed.

Suitable inorganic solvents include acidic solvents, alkaline solventsand/or water. Water is preferably employed as solvent in the solution.

The solution is preferably an aqueous solution.

Suitable solutes for the solution include organic compounds, biologicalcompounds and/or inorganic compounds.

Suitable organic compounds include hydrocarbons, such as aliphatic andaromatic hydrocarbons. Mixtures of two or more organic compounds may beemployed. The hydrocarbons may be straight chain, branched and/orcyclic. Examples of suitable hydrocarbons include, but are not limitedto, alkanes, alkenes and alkynes. The hydrocarbons may be substitutedwith one or more heteroatoms, for example, fluorine, chlorine, bromine,iodine, oxygen, sulphur, nitrogen, and/or phosphorus atoms. In oneembodiment, oxygenated hydrocarbons, such as aldehydes, ketones,carboxylic acids, ethers, esters, alcohols and/or their derivatives maybe employed. The organic solute species may have a molecular weight offrom 100 to 10000 gmol⁻¹, preferably, 300 to 5000 gmol⁻¹, morepreferably, 400 to 2000 gmol⁻¹ and, even more preferably, 500 to 1000gmol⁻¹.

Suitable biological compounds include proteins, amino acids, nucleicacids, carbohydrates and lipids. Mixtures of two or more biologicalcompounds may be employed. Preferred biological solutes include sugars,such as cane sugar and/or beet sugar. Glucose, fructose and sucrose mayalso be employed. The biological solute species may have a molecularweight of from 100 to 10000 gmol⁻¹, preferably, 300 to 5000 gmol⁻¹, morepreferably, 400 to 2000 gmol⁻¹ and, even more preferably, 500 to 1000gmol⁻¹.

Preferably, the solution is a solution of one or more inorganiccompounds, such as inorganic salts. Suitable salts include metal orammonium salts. Mixtures of two or more salts may be employed. Examplesinclude, but are not limited to, fluorides, chlorides, bromides,iodides, sulphates, sulphites, sulphides, carbonates,hydrogencarbonates, nitrates, nitrites, nitrides, hydrogenphosphates,phosphates, aluminates, borates, bromates, carbides, chlorides,perchlorates, hypochlorates, chromates, fluorosilicates,fluorosilicates, fluorosulphates, silicates, cyanides and cyanates.Preferably, salts of alkali and/or alkali earth metals are employed.Examples of such metals include, but are not limited to, lithium,sodium, potassium, magnesium, calcium and strontium.

Preferably, the solution is an aqueous solution of at least one saltselected from sodium chloride, potassium chloride, potassium nitrate,magnesium sulfate (e.g. MgSO₄.6H₂O or MgSO₄.7H₂O), magnesium chloride(e.g. MgCl₂.6H₂O), sodium sulfate (e.g. Na₂SO₄.10H₂O), calcium chloride(e.g. CaCl₂.2H₂O or CaCl₂.6H₂O), sodium carbonate, disodiumhydrogenphosphate (Na₂HPO₄.12H₂O) and potassium alum (24H₂O). In apreferred embodiment, the solution is an aqueous solution of sodiumchloride.

Preferably, the solution is formed by introducing a known quantity of asolute into a known quantity of solvent. Preferably, the solutionconsists essentially of a selected solute dissolved in a selectedsolvent. For example, in one embodiment, the process of the presentinvention further comprises the step of dissolving a selected solute ina selected solvent. In one embodiment, the solution is formed by mixingammonia and carbon dioxide in water. The resulting solution may containa concentrated solution of ammonia, carbon dioxide, ammonium carbonate,ammonium bicarbonate and ammonium carbamates as described in WO02/060825.

Alternatively, the solution may be derived from an existing stream suchas a waste stream from an industrial process. For example, the solutionmay be a cooling tower blowdown effluent, seawater, a water desalinationeffluent or an effluent from an oil extraction process.

In one embodiment, the solution has a solute (e.g. salt) concentrationof 1 to 400 weight %, preferably, 2 to 100 weight %, more preferably, 5to 80 weight %, for example, 10 to 50 weight %. The solute may be one ormore of the solutes mentioned above. For example, the solute may be asalt selected from sodium chloride, potassium chloride, potassiumnitrate, magnesium sulfate (e.g. MgSO₄.6H₂O or MgSO₄.7H₂O), magnesiumchloride (e.g. MgCl₂.6H₂O), sodium sulfate (e.g. Na₂SO₄.10H₂O), calciumchloride (e.g. CaCl₂.2H₂O or CaCl₂.6H₂O), sodium carbonate, disodiumhydrogenphosphate (Na₂HPO₄.12H₂O) and potassium alum (24H₂O).Alternatively, the solute may be a sugar.

In one embodiment, the solution is a sodium chloride solution having asodium chloride concentration of 2 to 39 weight %, preferably, 5 to 35weight %, more preferably, 10 to 30 weight %. In another embodiment, thesolution is a potassium chloride solution having a potassium chlorideconcentration of 5 to 50 weight %, preferably, 10 to 45 weight %, morepreferably, 15 to 35 weight %. In another embodiment, the solution is apotassium nitrate solution having a potassium nitrate concentration of 5to 80 weight %, preferably, 10 to 60 weight %, more preferably, 15 to 45weight %. In yet another embodiment, the solution is a calcium chloridesolution having a calcium chloride concentration of 5 to 120 weight %,preferably, 10 to 100 weight %, more preferably, 15 to 80 weight %. Inan alternative embodiment, the solution is a sodium carbonate solutionhaving a sodium carbonate concentration of 5 to 45 weight %, preferably,10 to 35 weight %, more preferably, 15 to 30 weight %. In a furtherembodiment, the solution is a disodium hydrogenphosphate solution havinga disodium hydrogenphosphate concentration of 5 to 39 weight %,preferably, 10 to 35 weight %, more preferably, 15 to 30 weight %. Inanother embodiment, the solution is a sodium sulphate solution having asalt concentration of 5 to 45 weight %, preferably, 10 to 40 weight %,more preferably, 15 to 39 weight %. In yet another embodiment, thesolution is a magnesium sulphate solution having a magnesium sulphateconcentration of 5 to 100 weight %, preferably, 10 to 80 weight %, morepreferably, 15 to 75 weight %.

The difference in TDS of the liquid and the solution may be at least 1weight %, for example, from 1 to 39 weight %, preferably, 5 to 35 weight%.

Any suitable selective membrane may be used in the process of thepresent invention. An array of membranes may be employed. Suitablemembranes include cellulose acetate (CA) and polyamide (PA) membranes.The membrane may be planar or take the form of a tube or a hollow fibre.Thin membranes may be employed. If desired, the membrane may besupported on a supporting structure, such as a mesh support.

In one embodiment, one or more tubular membranes may be disposed withina housing. The liquid may be introduced into the housing, whilst thesolution may be introduced into the tubular membranes. As the solventconcentration of the liquid is higher than that of the solution, liquidwill diffuse across the membrane into the solution. Thus, the solutionwill become increasingly diluted with liquid. The diluted solution maybe recovered from the interior of the tubular membranes, whilst theliquid may be removed from the housing.

When a planar membrane is employed, the sheet may be rolled such that itdefines a spiral in cross-section.

The pore size of the membrane may be selected depending on the size ofthe solvent molecules that require separation. The membrane may have anaverage pore size of 1 to 60 Angstroms, preferably, 2 to 50 Angstroms,more preferably, 5 to 40 Angstroms, for example, 10 to 30 Angstroms. Inone embodiment, the membrane has an average pore size of 12 to 25Angstroms.

It may be possible to use a membrane having a pore size that allows twoor more different types of solvent molecules to pass through themembrane. Conventional semi-permeable membranes may be employed.Typically, such semi-permeable membranes have an average pore size of,for example, 1 to 5 Angstroms.

The flow of solvent across a selective membrane is generally influencedby thermal conditions. Thus, the liquid and solution on respective sidesof the membrane may be heated or cooled, if desired. Preferably, thesolution is heated to a temperature of 30 to 90° C., preferably, 50 to70° C. The liquid may be cooled, for example, to −20° C. to 20° C., forexample, 7 to 12° C. Chemical reactions may also be carried out oneither side of the membrane, if desired. In one embodiment, the solutionand/or liquid may be agitated. In another embodiment, the solutionand/or liquid may be subjected to an external field, such as anelectrical, microwave and/or laser field, to enhance the osmoticpotential difference between the two solutions.

The process of the present invention may further comprise apre-treatment step of removing contaminants, such as suspended particlesand biological matter, from the liquid (e.g. a waste stream, seawater,river water, lake water or brackish water). Additionally oralternatively, a threshold inhibitor to control scaling may be added tothe liquid. Pre-treatment steps to alter the pH of the liquid may alsobe employed.

Optionally, the solution may also be treated to remove contaminants,such as suspended particles and biological matter. Additionally oralternatively, a threshold inhibitor to control scaling may be added tothe solution. Pre-treatment steps to alter the pH of the solution mayalso be employed.

Optionally, step a) of the process may be repeated one or more times.Thus, the pressurised solution from step a) may be positioned on oneside of a further selective membrane and a further solution may beplaced on the other side of the membrane. The further solution has ahigher osmotic potential than the solution on the other side of themembrane, such that the further solution becomes pressurised by theinflux of liquid across the membrane. The pressure of the furthersolution may be used to drive the prime mover.

According to a further aspect of the present invention, there isprovided an apparatus for driving a prime mover, said apparatuscomprising

-   -   a prime mover,    -   a housing comprising a selective membrane for separating a        liquid from a solution having a higher solute concentration than        the liquid and configured such that liquid passing through the        membrane pressurises the solution,    -   means for transmitting the pressure generated in the solution to        the prime mover,    -   means for recovering the solution,    -   means for separating solvent from the solution to produce a        residual product, and    -   means for recycling the residual product and/or the separated        solvent to the housing.

The residual product may be recycled to solution contained in thehousing of the apparatus. Alternatively, the residual product may berecycled to solution contained in the housing of another apparatusaccording to the present invention.

The prime mover may be any suitable device which is suitable forconverting energy in the solution into mechanical power. Suitable primemovers include rotary prime movers, such as turbines. Thus, the primemover may be used to generate power.

Alternatively, the prime mover may be or form part of a pressureexchange system. Thus, the prime mover may also be used to transferenergy from the pressurised solution to another fluid. Examples ofsuitable pressure exchange systems are described in U.S. Pat. No.4,887,942, U.S. Pat. No. 5,338,158, U.S. Pat. No. 5,988,993 and U.S.Pat. No. 6,540,487.

These and other aspects of the present invention will now be describedwith reference to the accompanying drawings, in which

FIG. 1 is a schematic flow diagram of a process according to a firstembodiment of the present invention,

FIG. 2 is a schematic flow diagram of a process according to a secondembodiment of the present invention, and

FIG. 3 is a schematic flow diagram of a process according to a thirdembodiment of the present invention.

Reference is first made to FIG. 1 of the drawings. This Figure depicts aprocess according to a first embodiment of the present invention. Theprocess is performed using an apparatus 10 comprising an osmotic cell12, a prime mover 14 (e.g. turbine coupled to an electrical generator)and a separator 16. The osmotic cell 12 comprises a semi-permeablemembrane 18.

In use, water 11 (e.g. seawater) is introduced to one side of themembrane 18. A 30 weight % solution of sodium chloride 13 is introducedto the opposite side of the membrane 18. As the sodium chloride solutionhas a sodium chloride concentration that is higher than the totaldissolved salt (TDS) concentration of seawater, water flows across themembrane 18 by osmosis. The influx of water across the membrane 18increases the pressure of the sodium chloride solution.

The pressurised sodium chloride solution is removed from the osmoticcell 12 and introduced to the prime mover 14. It is not necessary topump the sodium chloride solution as the solution is pressurised by theosmosis step. The pressurised sodium chloride solution is used to drivethe prime mover 14. The mechanical energy produced may be converted toother forms of energy, such as electrical energy.

The sodium chloride solution may then be removed from the prime mover 14and introduced into the separator 16. In the separator 16, water isremoved from the sodium chloride solution by evaporation. Once water isremoved from the sodium chloride solution, the sodium chloride solutionis recycled to the osmotic cell 12 for re-use. Thus, fresh sodiumchloride solution is not required to replenish or replace the sodiumchloride solution in the osmosis step.

The water removed by the evaporation step may be recovered and used, forexample, as drinking water. Thus, this embodiment of the presentinvention may be used to desalinate seawater.

The apparatus 10 is located in close proximity to a conventional powerstation 22. The power station 22 comprises a boiler 24, a prime mover 26(steam turbine) and a thermal unit 28 (condenser in power plant).

In use, water is introduced into the boiler 24 via a pump 30. The wateris heated in the boiler 24 by the combustion of fuel 32 to producesuperheated steam. The superheated steam is then introduced at highpressure to the prime mover (steam turbine) 26, and is used to drive theprime mover 26 to generate mechanical energy. The mechanical energy ofthe rotating prime mover 26 may be converted into other forms of energy,such as electrical energy.

Saturated or superheated steam is then recovered from the prime mover 26and introduced into the thermal unit 28. In the thermal unit, the steamis condensed to water. The excess heat from the steam is used toevaporate water from the sodium chloride solution of apparatus 10. Thus,the sodium chloride solution from the prime mover 14 of apparatus 10 isused as a coolant in the thermal unit 28 of the power station 22. Theseparator 16 of apparatus 10, therefore, is effectively the same as thethermal unit 28 of the power station 22.

Once cooled, the condensed steam of the power plant is recycled to theboiler 24 via the pump 30.

Removed steam or water from the sodium chloride solution by theseparator 16 can be used as a pure water product or recycled to unit 10.

Reference is now made to FIG. 2 of the drawings. This figure depicts aprocess according to a second embodiment of the present invention. Theprocess is performed using an apparatus 100. Apparatus 100 is similar tothe apparatus 10 of FIG. 1 and like components of the apparatus havebeen labelled with like numerals. Apparatus 100, however, is adapted foruse in cold climates. Thus, unlike the apparatus 10 of FIG. 1, theapparatus 100 comprises a separator 116 that is a crystallizer. In use,solution emerging from the prime mover 14 is introduced into theseparator 116 and cooled by the ambient temperature to produce ice and aconcentrated sodium chloride solution. The former is removed anddiscarded, whilst the latter is recycled to the osmotic cell 12.

Reference is now made to FIG. 3 of the drawings. This figure depicts aprocess according to a third embodiment of the present invention. Theprocess is performed using an apparatus 200. Apparatus 200 is similar tothe apparatus 10 of FIG. 1 and like components of the apparatus havebeen labelled with like numerals. Apparatus 200, however, is adapted foruse in warm dry climates. Thus, unlike the apparatus 10 of FIG. 1, theapparatus 100 comprises a separator 216 that relies on natural oreffective evaporation and/or solar energy to remove solvent from thesolution emerging from the prime mover 14.

1. A process for driving a prime mover, said process comprising a)positioning a selective membrane having an average pore size of at least10 Angstroms between a liquid and a solution having a higher osmoticpotential than the liquid, such that the solution becomes pressurised bythe influx of liquid across the membrane, b) using the pressuregenerated in the solution to drive a prime mover, c) recovering thesolution, d) separating at least some of the solvent from the solutionto form a residual product, and e) recycling the separated solventand/or the residual product of step d) to step a).
 2. A process asclaimed in claim 1, wherein the prime mover is a rotary prime mover. 3.A process as claimed in claim 1 or 2, wherein the prime mover forms partof a pressure exchange system.
 4. A process as claimed in any one of thepreceding claims, wherein the solution is an aqueous solution.
 5. Aprocess as claimed in any one of the preceding claims, wherein thesolution is solution of a salt selected from sodium chloride, potassiumchloride, potassium nitrate, magnesium sulfate, magnesium chloride,sodium sulfate, calcium chloride, sodium carbonate, disodiumhydrogenphosphate and potassium alum.
 6. A process as claimed in claim 4wherein the aqueous solution is formed by dissolving ammonia and carbondioxide in water.
 7. A process as claimed in claim 6, which is anaqueous solution of ammonia, carbon dioxide, ammonium carbonate,ammonium bicarbonate and ammonium carbamates.
 8. A process as claimed inany one of the preceding claims, wherein the solution has a soluteconcentration of 1 to 400 weight %.
 9. A process as claimed in any oneof the preceding claims, wherein the liquid is selected from the groupconsisting of freshwater, seawater, brackish water and a waste streamfrom an industrial or agricultural process.
 10. A process as claimed inany one of the preceding claims, wherein the liquid is or comprises thesame solvent as the solvent of the solution.
 11. A process as claimed inany one of the preceding claims, wherein solvent is removed in step d)by a thermal and/or membrane separation method.
 12. A process as claimedin claim 11, wherein the solvent is removed using a method selected fromevaporation, distillation and crystallization.
 13. A process as claimedin claim 12, wherein the solvent is removed by at least one methodselected from multi-stage flash distillation, multi-effect distillation,mechanical vapour compression and rapid spray desalination.
 14. Aprocess as claimed in claim 11, wherein the solvent is removed by atleast one method selected from ion-exchange, electrodialysisnanofiltration and osmosis.
 15. A process as claimed in any one of thepreceding claims, wherein the energy required to remove solvent in stepd) is provided by the wind power, thermal energy of the surroundingenvironment, solar energy, geothermal energy, energy from a biologicalprocess, energy from the combustion of fuel and/or excess heat frompower plants and other industrial processes.
 16. A process as claimed inany one of the preceding claims, wherein at least some of the solventrecovered in step d) is recycled to a liquid for step a).
 17. A processas claimed in any one of the preceding claims, which comprises using thepressure generated in the solution to transfer the solution to anelevated location, and using the potential energy of the elevatedsolution to drive the prime mover.
 18. A process as claimed in any oneof the preceding claims, wherein the solution from step a) istransferred to an elevated height where the ambient temperature is (i)low enough to crystallize at least some of the solute in the solution,or (ii) below the freezing point of the solvent to crystallize thesolvent, such that the solution is separated into a portion having a lowsolute concentration and a portion having a high solute concentration.19. A process as claimed in claim 18, wherein each of said portions isreturned to ground level, such that the potential energy of each of theportions can be used to drive the prime mover.
 20. A process as claimedin any one of the preceding claims, wherein the thermal energy requiredto separate the solvent from the solution is step d) is provided by thecompression and decompression of gas.
 21. A process as claimed in anyone of the preceding claims, wherein the selective membrane of step a)has an average pore size of 10 to 60 Angstroms, preferably 12 to 50Angstroms.
 22. An apparatus for driving a prime mover, said apparatuscomprising a prime mover, a housing comprising a selective membranehaving an average pore size of at least 10 Angstroms for separating aliquid from a solution having a higher solute concentration than theliquid and configured such that liquid passing through the membranepressurises the solution, means for transmitting the pressure generatedin the solution to the prime mover, means for recovering the solution,means for separating solvent from the solution to produce a residualproduct, and means for recycling the residual product and/or theseparated solvent to the housing.